Scan gated television cameras, for example, vidicons, orthicons, and solid state arrays are employed in a number of applications wherein the exposure time of the camera (the time between scanning) is increased to relatively long times to compensate for low light levels or to detect low light emissions. For example, in viewing of microscopic living cells through a microscope lens system, low light levels may be required to prevent or delay light-induced changes in the cells. It may also be desirable to detect and image low levels of light emission from the cells, such as phosphorescence. In such circumstances, the camera shutter is "opened" (by scanning the image sensor to discharge its surface) and light is allowed to impinge on the camera's photosensitive surface for a longer period than is normal, for example, for a full second. This in effect integrates the light received by the camera over that second, and the scan signal output "takes" an image which is the result of the long time exposure. Thus, as with a long time exposure of a photographic camera in a dark room, a useful image can be produced.
This method of operating a television camera is termed in the art "long term target integration," and control systems are commonly available for operating the camera in this way. One such prior art system is that commercially available from the assignee of this invention, DAGE-MTI, Inc., and known as the SIT66G Model and what is described in Service Manual No. 970268-05 published by DAGE-MTI, Inc. in November, 1988. (A copy of this publication should be available in the Technical Library of the Patent and Trademark Office and should be found in the file wrapper of this patent.)
A problem in this and other known such television camera systems which are used in long term target integration is that the black picture level current increases during integration and it is difficult or impossible to be determined in advance. The difficulty in determining or predicting this level is that it results from the combination of the image sensor dark current which is highly temperature dependent, stray background illumination such as camera filament leakage and light scatting within the optical imaging system, and perhaps other factors which can vary from one application to another. Nevertheless, it is desirable to attempt to counteract this increase in black level so as to produce a picture with good contrast and detail.
In previous systems, this compensation was achieved by a trial and error approach. That is, by setting the picture black level to a predetermined position which is lower than the normal non-integrated setting, then integrating for the period desired, reading, storing and viewing of the resultant video picture information, and then resetting the picture black level control slightly toward the anticipated correct setting and repeating this process until a satisfactory picture black level is achieved. This process is, of course, inexact and time consuming.